Silicon-Controlled-Rectifier (SCR) devices are widely used for electrostatic discharge (ESD) protection because they have smaller size, lower holding voltage and lower capacitance than bipolar transistors. The smaller size is due to the higher current-carrying capability compared to bipolar transistors. The lower holding voltage is due to the interaction of the NPN and the PNP transistors in the SCR devices. The lower capacitance is due to the device structure and the smaller size.
Because of the smaller size, the SCR devices can be used to protect high-pin-count integrated circuits without significant die-size penalty. Because of the lower holding voltage, the power dissipation during an ESD event is minimized. This is critical for discharging the destructive current. Because of the lower capacitance, the SCR devices can be used to protect high-speed pins.
Unfortunately, SCR devices are known to be slower than bipolar transistors. As a result, while SCR devices are capable of protecting integrated circuits against the Human-Body-Model (HBM) ESD, they are not effective against the Charge-Device-Model (CDM) ESD which is much faster. Moreover, SCR devices are able to discharge pulses in only one direction.